Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which brightness of a pixel is based on intensity of the echo return. In a so-called "color flow" mode, the flow of blood or movement of tissue can be imaged. Conventional ultrasound flow imaging methods use either the Doppler principle or a time-domain cross-correlation method to estimate average flow velocity, which is then displayed in color overlaid on a B-mode image.
Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The frequency shift of backscattered ultrasound waves may be used to measure velocity of tissue or blood. The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer. The Doppler shift may be processed to estimate the average flow velocity, which is displayed using different colors to represent speed and direction of flow. The color flow velocity mode displays hundreds of adjacent sample volumes simultaneously, all color-coded to represent velocity of each individual sample volume.
Conventional ultrasound flow imaging displays either average Doppler power ("power Doppler imaging") or average flow velocity ("color flow velocity imaging") as a color overlay on a B-mode image. The transmitted pulses are typically more narrow-band than B-mode pulses in order to gain Doppler sensitivity. Operating on a packet of as many as 16 transmits, a high-pass wall filter first rejects echoes from slower-moving tissue or vessel walls to reduce the signal dynamic range. The number of wall filter output samples per packet is given by (N-W+1), where N is the packet size and W is wall filter length. Subsequently, instantaneous Doppler power is computed as the magnitude squared of each wall filter quadrature output signal, and the average of all output signals yields the average Doppler power. Alternatively, the average velocity is computed from the wall filter quadrature output signal based on the Doppler principle (phase change) or time delay between firings. The Kasai autocorrelation algorithm or a time-domain cross-correlation algorithm can be used to estimate the average flow velocity.
Although conventional color-flow imaging has very good flow sensitivity, the ability to see physical flow is limited by its limited dynamic range (which is partially dependent on the compression curve), limited resolution (due to narrow-band pulses), limited frame rate (due to large packet sizes), and axial-only flow sensitivity (which is dictated by the reliance on the Doppler effect). In addition, conventional color-flow imaging suffers from artifacts such as aliasing, color blooming and bleeding.
In medical diagnostic ultrasound imaging, it is also desirable to optimize the signal-to-noise ratio (SNR). Increased SNR can be used to obtain increased penetration at a given imaging frequency or to improve resolution by facilitating ultrasonic imaging at a higher frequency. Coded excitation is a well-known radar technique used to increase signal-to-noise ratio in situations where the peak power of a transmitted signal cannot be increased but the average power can. This is often true in medical ultrasound imaging, where system design limitations dictate the peak amplitude of the signal driving the transducer. In this situation, longer signals, such as chirps, can be used to deliver higher average power values, and temporal resolution can be restored by correlating the return signal with a matched filter. Chirps, however, are expensive to implement on a phased array ultrasound system due to the complexity of the electronics, so binary codes, or codes that can be easily represented digitally as a series of digits equal to +1, -1 or 0, are much more practical. Binary codes are also preferred because they contain the most energy for a given peak amplitude and pulse duration.
A method for imaging moving blood reflectors using binary codes and displaying a combination of the flow image and the tissue image without overlay has been disclosed in the parent (Ser. No. 09/299,034) of the present application. One method of flow imaging disclosed uses single-transmit (e.g., Barker) codes. However, single-transmit codes have range lobes and require a long mismatched decoding filter. Consequently, single-transmit codes cannot be used on lower-frequency probes if the decoding filter length in the hardware is insufficient.
There is a need for a way of achieving flow imaging which will alleviate the limitations of the single-transmit codes and which can be employed with all types of probes.